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Just as we near the end of the hurricane season in the Atlantic Ocean, winds whirl and clouds churn
2 billion miles away in the atmosphere of Uranus, forming a dark vortex large enough to engulf
two-thirds of the United States.

Lawrence Sromovsky of the University of Wisconsin-Madison leads a team that used NASA's Hubble
Space Telescope to take the first definitive images of a dark spot on Uranus. The elongated feature
measures 1,100 miles by 1,900 miles (1,700 kilometers by 3,000 kilometers).

There have been prior unconfirmed sightings of dark spots on Uranus, including sketches made in the
early 1900s, low-contrast ultraviolet Voyager spacecraft flyby images in 1986, and near-infrared
observations taken from a ground-based observatory in 1993. However, no other Hubble images taken
almost every summer from 1994 through early 2006 have shown such a dark spot. This indicates that the
current dark disturbance probably formed very recently, researchers said.

Although rare on Uranus, dark spots have been frequently observed on Neptune. Uranus is
similar in size and atmospheric composition to Neptune, but it has not appeared to have as active an
atmosphere. Recently, however, Uranus's atmosphere has shown an increase in activity.

The development of a dark spot may be a signal of the oncoming uranian northern spring, said researchers.
Uranus is approaching its December 2007 equinox, when the Sun will shine directly over the equator.
"We have hypothesized that Uranus might become more Neptune-like as it approached its equinox," said
team member Heidi B. Hammel of the Space Science Institute in Boulder, Colo. "The sudden appearance
of this unusual dark feature suggests we might be right."

The dark spot was detected at a latitude of 27 degrees in Uranus's northern hemisphere, which is
just now becoming fully exposed to sunlight after many years of being in shadow. Astronomers are
keenly interested in how strongly and quickly the atmosphere of Uranus seems to be responding to
seasonal sunlight changes.

Uranus's rotation axis is tilted almost parallel to its orbital plane, such that the planet appears to be
rotating on its side. This sideways orientation leads to extreme seasons during the planet's 84-year path
around the Sun.

This three-wavelength composite image was taken with Hubble's Advanced Camera for Surveys on
August 23, 2006. The research team found the dark spot again on August 24. The inset image shows a
magnified view of the spot with enhanced contrast. Uranus's north pole is near the 3 o'clock position in
this image. The bright band in the southern hemisphere is at 45 degrees south.